Hitherto, electroluminescent displays (ELD) have been used as light emission type electronic displaying device. As the element constituting the ELD, inorganic electroluminescent elements and organic electroluminescent elements, hereinafter referred to as organic EL element, can be cited.
The organic EL element has a constitution in which a light emission layer is placed between a cathode and an anode and the element emits light by utilizing the light emission (fluorescence or phosphorescence) caused by quenching of exciton generated by recombination of an electron and a positive hole injected from the electrodes to the light emission layer. The organic EL elements can emit light by applying a voltage of from several volt to several ten volt, and such the element is noted from the viewpoint of that the element gives wide viewing angle and high visibility because the element is self light emission type and further gives high portability and space saving since the element is a thin layer type complete solid state element.
Recently, an organic EL element using phosphorescent light emitted from the triplet excited state is reported by Princeton University; cf. Non-patent Publication 1, and the investigation on the material emitting phosphorescence at room temperature is accelerated; cf. Non-patent Publication 2 and Patent publication 1 for example. When the triplet excited state is utilized, the upper limit of the internal quantum efficiency becomes 100%. Therefore, the light emission efficiency is theoretically raised by four times of that of the singlet excited state so that performance almost the same as that of a cathode ray tube can be obtained. Consequently, such the element is noted since which can be used for purpose of lighting. For example, synthesizing of many compounds principally heavy metal complexes such as iridium complexes have been carried out; cf. Non-patent Publication 3 for example.
At present, investigation for further rising the light emission efficiency and the life timer thereof is carried out; cf. Patent Publications 2 and 3 for example. Though a taking out efficiency of green light of 20% which is theoretical limit of the efficiency can be attained, sufficient taking out efficiency cannot be obtained regarding the other color light and improvement is required. Particularly, an element capable of emitting blue light with high efficiency is required and a compound having larger band gap is demanded.
The organic light emitting element realizing high luminance light emission is an element constituted by laminating organic substance layers by vapor deposition method, however, a production by coating method is disclosed for simplifying the production process, rising the processing suitability and increasing the product area; cf. Patent Publication 4 for example.
However, improvement in the voltage rising during the constant electric current driving and occurrence of dark spots, and durability under high temperature and high humidity condition is required to the conventional organic EL elements. Further improvement in the luminance of emitted light is also demanded.
Patent Publication 1: U.S. Pat. No. 6,097,147
Patent Publication 2: Tokkai 2002-100476
Patent Publication 3: Tokkai 2002-117978
Patent Publication 4: Tokkai 2002-299061
Non-patent Publication 1: M. A. Baldo et al., Nature, 395 pp. 151-154 (1998)
Non-patent Publication 2: M. A. Baldo et al., Nature, 403, 17, pp. 750-753 (2000)
Non-patent Publication 3: S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001)